1. Field of the Invention
The present invention relates generally to the field of magnetic storage, and magnetic sensor devices. More particularly, to the application of magnetic recording read sensor, magnetic random access memory, and magnetic field sensors, and the manufacturing method of producing mass quantity of such devices by wafer level process.
2. Relevant Background
In normal conductors, diffusion is primarily responsible for the electron transport. Certain conductive types of carbon nanotubes are found to conduct electrons via ballistic transport. That is, the electrons are transported through the entire length of a nanotube without scattering, thus maintaining their original quantum state. This gives rise to the possibility of these types of nanotubes being utilized in devices, whose operating principle is based upon spin-polarized electron transport. Nanotube-based devices offer the potential of significantly lower power consumption, faster switching speed. Metallic nanotubes have the ability to carry a large current density of 109 A/cm2, which is 1000 times higher than the copper wire. This, along with the superior heat conductivity and temperature stability, can potentially enhance the long term reliability of nanotube-based devices. In addition, the ballistic transport of spin polarized electrons through the nanotubes eliminates the need for intimate contact between the magnetic electrodes, therefore offers the possibility of physically separating the pinned and free layers in a spin-valve type device.
In prior art, a CPP (current-perpendicular-to-plane) spin valve device consists of a pinned and a free ferromagnetic layer separated by a spacer layer. For a giant magnetoresistive (GMR) CPP device, the spacer layer is typically made of copper (Cu); whereas, a spin-dependent tunneling device, a tunneling barrier of alumina (AlOx) or of other metal oxide (HfOx, HfAlO, ZnOx) is used. Upon application of a bias voltage, the electrical current passes from one electrode to the other via the nonmagnetic spacer layer. The resistance of the device is a function of the relative magnetic configuration between the two ferromagnetic layers. The output of the device, which is proportional to the resistance change, is thereby dependent upon the free layer magnetization direction, and is modulated by the signal magnetic field. The physical resolution of the read sensor along the data track direction is determined, in part, by the thickness of the spin-valve film at the air bearing surface (ABS). Therefore, a reduction in the thickness of the sensor film, which is exposed at the ABS, would result in improvement in linear resolution of the read sensor.
In prior art, the use of nanotubes as current channels has been demonstrated in field effect transistor (FET) devices, as the nanotubes are randomly disposed between the source and drain electrodes. This is typically achieved by fabrication of source and drain electrodes using conventional semiconductor manufacturing process (photolithography and patterning), followed by random deposition of nanotubes on the same substrate, and relied on chance that a nanotube with the proper alignment would be found. Another alternative method that has been used in prior art is to deposit nanotubes on a substrate first. This is followed by pattern imaging and recognition using scanning electron microscopy, which was then used to guide the fabrication of contact leads around individual nanotube via e-beam lithography. Both of these techniques are not viable approaches of producing large quantity of nanotube-based devices. Both methods are disadvantageous because of the lack of control over the precise positioning and alignment of nanotubes with respect to the rest of device structure, and the lack of ability to select nanotubes of proper diameter, length, and electrical properties (metallic or semiconductive).
In prior art, the methods of growing regular array of vertically oriented nanotubes have been shown. One of these methods involves controlled growth of carbon nanotubes on pre-patterned dots of catalyst. Deposition of nanotubes into channels of anodized aluminum has also been used. Both methods can be used to fabricate vertically aligned nanotubes.
It is shown in the aforementioned prior arts that in-plane or vertically aligned nanotubes may be used as channels connecting electrodes made of conventional metals. In conventional spin-valve type devices, the pinned and free layer may be considered as ferromagnetic electrodes, which are in such intimate contact through a spacer layer that the film stack is normally fabricated in a continuous sequence of deposition steps. The linear resolution of a read head utilizing spin-valve sensor can be improved by physically separating the pinned and the free layer, and inserting only the free layer between the shields, thereby significantly reduce the read gap length. Other advantages of having separated pinned and free layer include reduced magnetic interaction between them, as well as increased ability to optimize the properties of the layers individually. The physical separation of the pinned and free layer is only possible if there is a ballistic transport mechanism between the two layers, which can be realized by incorporating nanotube channels. Therefore, there is a need in the art for designs and methods of fabrication of carbon nanotubes into spin-valve type electronic devices.